Thermoplastic polyolefins (“TPO”) are widely used in membrane applications. In roofing applications in particular, the use of TPO-based membranes is rapidly growing. Most TPO roofing membranes include a fabric or scrim coated on both sides with polymer compounds based on polypropylene reactor copolymers. The top layer, which is often white, is formulated with non-halogen flame-retardants (predominantly magnesium hydroxide), UV stabilizers, antioxidants and pigments (titanium di-oxide). The bottom layer is generally polymer-rich. The white top layer reflects sunlight and prevents thermal heating inside the building, thereby reducing air conditioning costs. Ability to formulate white compounds is an advantage for TPO over some other roofing systems. TPO membranes are heat welded directly on the roof to form large sheets. This process eliminates the need for expensive solvent-based adhesives that are commonly used in other single-ply roofing. Because of the widespread use of TPO membranes in roofing and other applications under environmentally harsh conditions, it is particularly desirable to improve membrane properties, such as strength and tear resistance.
Blends of metallocene catalyzed plastomers with polypropylene in membrane applications are known. For example, EXACT™ 0201 and EXACT™ 8201 (metallocene-catalyzed ethylene-octene plastomers having densities of 0.902 g/cm3 and 0.882 g/cm3, respectively, available from ExxonMobil Chemical Co., Houston, Tex.) have been used in blends with a polypropylene impact copolymer to form membranes suitable for roofing applications. (N. Dharmarajan, T. C. Yu and D. K. Metzler, “Metallocene Plastomer Based Thermoplastic Olefin Compounds for Roof Membrane Applications”, Society of Plastics Engineers (SPE) ANTEC 2001 Meeting, May 2001, Dallas, Tex.) The plastomer-polypropylene compositions are said to have a good balance of mechanical properties, notably tensile strength and tear resistance, as well as other desirable properties.
Other background references include WO 92/14784; WO 98/54260; Patent Abstracts of Japan, vol. 2000, no. 05 (2000-09-14) & JP 2000 063581 (2000-02-29); Patent Abstracts of Japan, vol. 1996, no. 02 (1996-02-29) & JP 07 278377A (1995-10-24); EP 0 850 756; Patent Abstracts of Japan, vol. 1997, no. 03 (1997-03-31) & JP 08 311271A (1996-11-26); and U.S. Pat. No. 5,006,383.
It would be desirable to have polymer blend compositions suitable for membrane applications, wherein the membrane would have further improved tensile strength and tear resistance. Improved tensile strength and tear resistance would provide superior wind uplift resistance to the olefin membrane (resistance to high velocity winds) and the potential to downgauge to reduce composite thickness. It would further be desirable to provide polymer blend compositions suitable for membrane applications, wherein the compositions can be formulated at lower cost, without sacrificing ultimate membrane properties.